The inventions described and claimed herein pertain in general to electronic circuits, and more specifically, to phase shifters and phased array antennas. Since the original concept of phased array antennas was postulated, many attempts have been made to develop phase shifters using various technologies and circuit architectures. The presently described inventions comprise novel combinations of previously used individual phase shifter elements and architectures that are able to realize the best features and advantages of each technique.
The following terms used in this patent document have generally accepted meanings in electrical engineering literature and will not be specifically defined herein: resistance, component, circuit, electrons and electronic, capacitor and capacitive, inductor and inductive, control, signal, voltage, current, power, energy, frequency, Hertz, Megahertz (MHz), Gigahertz (GHz), radio-frequency (RF), microwave, millimeter-wave, and all terms of the S.I. and English unit systems.
Other terms have meanings in the context of this document that will be made clear. The description of the drawings and detailed description portions of this document include some of these precise terms that describe numbered elements of the drawings as they occur in the text. For the purposes of this patent document, the following terms are now defined:
Electronically controlled refers to a component, element, or circuit that changes its state or function when an electrical control signal is applied or varied. These signals can include voltage, current, frequency, phase, and other electrical characteristics. An electronically controlled device or element can be either “analog” or “digital”.
Digital refers to a component, element, or circuit having separate states that are discretized into an integer number of possibilities.
Analog refers to a component, element, or circuit able to exist in any one possible state out of a continuous range of possible states.
Reactance is an electrical property, which, together with resistance, helps define the circuit performance of a component. Reactance may be either capacitive or inductive. A capacitor component, for example, typically has large capacitive reactance and low resistance.
A varactor is a component that has a variable reactance. An electronically controlled varactor is therefore a component with a capacitance or an inductance value that can be changed by the application or variance of an electrical control signal.
Phase is an electrical property, which, together with amplitude, helps define the state of an electrical signal. Phase is used to describe time in time-varying, cyclical electrical signals.
A phase shift is a change of the phase state for a cyclical electrical signal. Phase shift occurs naturally over time, but it is electronically controlled phase shift is significant with regard to the inventions described.
A circuit element, or element refers to a small electrical circuit comprising one or more electronic components that performs an engineered electrical function.
A phase shift element is a circuit element that causes a phase shift of an electrical signal.
A phase shifter is an electrical circuit comprising one or more phase shift elements, and may also incorporate other engineered electrical functions. Our inventions are directed to various arrangements for phase shifters.
A μm, micron, or micrometer is a unit of length equal to one-one-thousandth of a millimeter
Microfabrication is a fabrication method of defining components delineated through photolithographic techniques made popular by the integrated circuit developer community.
Micromachining is the action of delineating a microfabricated element that has been photolithographically defined, often performed by an etching process using acids or bases.
MEMS and MEMS devices are Microfabricated ElectroMechanical Systems, which denotes a manufacturing technology that uses microfabrication techniques to develop miniaturized mechanical, electromechanical, and thermomechanical components. MEMS devices in this context typically refers to electrical actuators such as switches, relays, and varactors.
Phase shifter circuits and phased array antenna systems employ a variety of technologies and architectures in order to impose phase shifts on load signals. Phase shifters have typically been designed to be individual electronic circuit products that have one input for a load signal, have an arrangement for receiving digital or analog control signals to control the phase shift imposed on the load signal, and have an output for the load signal. In nearly every case, the load signal is an RF, microwave, or millimeter-wave signal that is part of a communications or radar system.
Phased array antenna systems implement phase shifter circuits and other phase shifting techniques in antennas that are used in RF, microwave, and millimeter-wave systems. A phased array antenna system architecture might not employ any phase shifter circuits. However, typically, it would employ tens to tens of thousands of phase shifter circuits. Designers of both phase shifter circuits and phased array antenna systems, therefore, consider phase shift inventions and technologies to be highly relevant to the advancement of radar and communications technologies.
The “phase” of an RF, microwave, or millimeter-wave signal defines the manner in which overlapping signals constructively or destructively interfere with each another. The precise control of load signal phase from a number of separate antennas enables the radiated signal to be electrically aimed in varying directions without having to physically aim the antenna. This ability to aim provides numerous functional advantages including but not limited to power consumption, size, speed, and functionality to radar and communications systems that employ phased array antennas. Because of the technical advantages of phased array antennas, improving phase shifter circuit designs and phased array antenna concepts has been the subject of industry research for many years.
A typical phase shifter circuit imposes a phase shift upon a load signal when a control signal is applied to the phase shifter. The amount of phase shift provided depends upon the control signal and upon the complexity and architecture of the phase shifter. The phase shift imposed typically varies between 0 (zero) and 360 degrees for communications systems, where the zero phase shift will be relative to some baseline phase shift. For an input phase at zero degrees, one phase shifter circuit might have baseline phase shift of 63 degrees at a particular frequency, and might therefore provide an output that has a phase shift of between 63 degrees and 423 degrees (360+63=423). Radar systems often have much greater phase shift requirements, so phase shifter circuits designed for these systems might provide between 0 and many thousands of degrees.
The majority of phase shift technologies and architectures do not provide a constant phase shift as a function of frequency. A desired phase shift may only be available for a limited frequency range, and operation outside of that frequency range will not generate a desired phase shift. Many phase shift technologies and architectures provide a phase shift that is linearly related to frequency; such a phase shifter circuit might provide 180 degrees of maximum phase shift at 12 GHz and only 90 degrees of maximum phase shift at 6 GHz.
There are many classification techniques that can be attributed to the variety of phase shift technologies and architectures in use by the phase shift circuit design and phased array antenna system fields. Within the context of this patent document, all of these technologies and architectures are considered in terms of the precision with which a desired phase shift can be imposed upon a load signal. There are two categories of technologies; those that provide a finite number of discrete phase shift options, and those that provide an infinite or continuous number of phase shift options. Finite-option phase shift technologies and circuit architectures are referred to as “digital” phase shift techniques, and infinite-option phase shift technologies and circuit architectures are referred to as “analog”. These definitions for the terms digital and analog with respect to defining inherent properties of technologies and architectures remain valid regardless of the type of electrical control signals used to control those digital and analog technologies and architectures. For example, an analog technology or architecture is always considered to maintain analog characteristics and definition in context of this patent document, even if that technology or architecture in a particular embodiment of this invention were to be controlled electrically by a “digitally-generated” analog voltage variance of a control signal.
Digital phase shift techniques have a discrete number of possible phase shift states, such that only a finite number of phase shift options are available for use. In a similar fashion as the computer industry, these phase shift states are often referred to in terms of the number of options, stated to the second power in terms of bits. A “three-bit phase shifter” would therefore be a digitally-controlled phase shifter circuit with a total of eight possible states (2^3=8). A “five-bit phase shifter” would have 32 possible states, and so forth. The values of these states are usually within a range of zero (relative to baseline, which could be any value) to a predetermined maximum phase shift. A 360 degree three-bit phase shifter, for example, would have the following degree phase shift options: 0, 45, 90, 135, 180, 215, 260, and 305 degrees. In the context of many systems, 360 degrees and 0 degrees are electrically equivalent, so the phase shifter would not require a full 360 degree phase shift.
Analog phase shift techniques have a continuous range of phase shift possibilities that can be imposed upon the load signal. The ability to impose precise values provides analog phase shift techniques important advantages over digital phase shift techniques. Continuous values of phase shift, rather than discrete values, provides a system designer with a greater range of options for implementing the phase shifter circuit, which means that antennas can be aimed more accurately, or the phase shifters themselves can compensate for manufacturing or environmental phase shift error. Analog phase shift technologies tend to have a limited range of applied phase shift, however, so multiple phase shift elements might be connected in series in order to obtain a phase shift range up to 360 degrees or more.
Analog phase shift technologies and architectures generally have a high signal loss characteristic, which means that the load signal can have a continuous, precise phase shift, but the majority of the load signal strength will be dissipated within the phase shifter circuit. In electrical engineering terms, the loss figure of phase shift technologies and circuit architectures is given in terms of decibels (dB), where a packaged 360 degree analog phase shifter might have between 6 and 12 dB of loss (losing ¾ to 15/16 of the signal power) at microwave frequencies. Digital architectures, on the other hand, might have 1.5 to 3 dB of loss per bit of phase shift precision at microwave frequencies.
In the fields of phase shifter circuit design and phased array antenna systems, there are many devices which incorporate different digital or analog phase shift architectures and technologies. Digital phase shift architectures are used to provide a large phase shift at a low circuit loss, and often have high linearity and other desirable electronic circuit performance qualities. As digital phase shifter designs increase in precision and resolution, however, they suffer high loss and complexity. Analog phase shift architectures and technologies are used to provide fine resolution and continuous tuning of circuits, but all known analog phase shift technologies and architectures suffer from very high loss.
Digital phase shift technologies have been developed which include a wide variety of semiconductor phase shifter products based on gallium-arsenide substrates and other traditional high-frequency integrated circuit technologies. Research is being carried out with respect to micromachining and MEMS technologies in an effort to develop digital phase shifter circuit architectures based on MEMS devices. One line of investigation is pursuing traditional architectures using MEMS devices (see U.S. Pat. No. 6,653,985), or employ MEMS devices in novel architectures within the phase shift circuits and antennas themselves (see Sikina—Published U.S. Patent Application 2003/0184476 A1 and Huff—Published U.S. Patent Application 2003,0020173 A1). The arrangements described in these publications have the same inflexibility shared by all digital phase shift circuit technologies.
Because of their tunability and precision advantages, analog phase shift technologies have been an active area of research and development by the industry. Developments range from using traditional semiconductor technologies (see U.S. Pat. No. 4,837,532), which have become commercial products over the years, to the use of modern materials as varactors for higher performance phase shifter circuit designs, which is the focus of Babbitt—U.S. Pat. No. 5,334,958, Osadchy—U.S. Pat. No. 6,621,377, and others—U.S. Pat. No. 6,538,603. Efforts continue in industry, government, and academia to create higher-performance, lower-loss analog phase shift circuitry.
One line of research has recognized some advantages of combining digital phase shift technologies with analog phase shift technologies in order to achieve both low loss and continuous tunability. Sayyah (U.S. Pat. No. 6,509,812) has designed an optical-electrical hybrid phase shifter, which uses MEMS devices for a low-loss digital (referred to as “coarse”) phase shift element in a first stage and an optically-controlled variable resistor for a continuously-tunable analog (referred to as “fine”) phase shift element in a second stage.
Despite the demonstrated long-felt need and the active and wide-ranging efforts by numerous researchers and groups including those above, none of the resulting devices embody the fundamental conflicting desired attributes of low loss and fine tunability of phase shifting in high-performance radar and communication systems.